Hi!
The first answer to your question you knew already. The human eye is
capable of seeing any shade of gray. I just wanted to get the "duh" answer
out of the way before I get into the stickier one.
Things get harder to answer when you ask how many shades of gray the human
eye can discriminate. Then the question is less how many shades can we
see, but, if given a choice, would we be able to identify the darker gray?
There are many ways to test this, because there are many different
conditions under which you could test gray-scale discrimination. Let's
assume that these are ideal conditions -- a bright room preferably lit with
sunlight -- fluorescent, halogen and incandescent lights are not true
"white" light, they're each weighted towards different parts of the
spectrum. Sunlight isn't either, but it's generally closer.
Then the question becomes how much change in "brightness" of the gray do
you need before you can tell that two shades of gray are different? We'll
treat brightness as a percent value, ranging from 0% (black) to 100%
(white). Most humans need a 1% change in brightness to discriminate
between two shades of gray (they get a 1% change right significantly more
than half of the time, which is how often you'd get it right if you were
guessing). So that would be about 100 different shades of gray.
Of course, we are seldom in ideal viewing conditions, so that number is
lower in an average viewing environment. How much lower depends, of
course, on the environment. I wish I had a reference to refer you to, but
I couldn't find any. My source is a professor who studies vision here at
the university.
If you have any other questions, please contact me.
Brenda